Indigo Power Cut

I want to build a home alarm system based on door and motion sensors. Essentially when the system is armed and one of the sensors is triggered a siren will go off.

However I was wondering what would happen if there is a power cut and the indigo server is down. Will my system still trigger the alarm.

The sensors are all battery operated.

It will not work.
If everying goes through Indigo, alarm will not trigger because Indigo and its interfaces will not receive and signal from sensors

So the only way to make this work is to add a UPS to the server? Otherwise it's kind of a useless home alarm system.

Exactly

BTW, I also have a very basic alarm system with motion sensors+door/window sensors.
Of course, if power failes, then you do not have any alarm system anymore

Indigo is not designed to be an alarm system. There are inexpensive alarm panels out there, and many of them can be integrated with Indigo.

So the only way to make this work is to add a UPS to the server? Otherwise it's kind of a useless home alarm system.

AND on the alarm siren and/or router-cablemodem/wireless AP/satellite uplink you are using to alert you. As others have said, Indigo CAN do these things, but there are a number of cheap alternatives that are DESIGNED just for that purpose, most of which can be coerced to send some kind of signal to indigo when things go wrong; with the caveat that unless the controller and alert system are battery backed, they're just as much "boat anchors" as the indigo server if the bad guys knock out the power somehow.

Well considering that the door sensors, motion sensor and alarm siren are all batter operated, the only vulnerability is the Indigo Server. If I put a UPS on that and my internet router then I should be fine?

I'd agree with flyingdiver, a dedicated stand alone alarm with a connection to Indigo would be the most secure option. I personally use a DSC alarm system with a serial link to indigo. It's fully independent so should indigo/the server fail the alarm will still work!

If the sensors and siren are Z-wave, Yes (although I've never heard of a battery powered Z-wave siren). If they are insteon, not so much; the powermodem can't be on the ups because that would block the powerline signals, and it's the thing that receives the sensor information and then sends the start command to the siren... although thinking about it, it might be possible to put an insteon range extender on the UPS and then have indigo directly link the sensors to the siren... in which case the server wouldn't have to be up... just the range extender. if it's Z-wave though, everything would have to pass through the server.

[jay (support)]

Everspring makes a siren that's both mains powered and can run on batteries.

I have Indigo connected to my alarm system. Indigo doesn't control it, but it does integrate with it. I can arm/disarm, see status, etc.
I have a dedicated UPS on my mac mini that gives about 1 hour of runtime. If your power goes out, most alarm systems have a battery backup too, but that battery will also run out.
I also have a UPS on my modem and router, and every essential device in my house.
Whatever you do, DO NOT BUY Cyberpower UPS's. They are pure garbage. I bought several for my office and home and they don't last long, some even had dead batteries after a week of usage. Just garbage. APC is infinitely better. They are priced about the same. So no reason to buy cyberpower.

Everspring makes a siren that's both mains powered and can run on batteries.

And the battery last forever on this thing.

Aeotec siren also has a 3 hour battery.
It's been available for almost two years now, too bad Indigo still doesn't support this device.